Fm-cw radar range system

ABSTRACT

5. A continuous wave radar system comprising: a source of sine wave signals; first means for frequency modulating a carrier frequency with said sine wave signals to provide a first frequency modulated signal; means for transmitting said first frequency modulated signal; second means for frequency modulating a carrier frequency with said sine wave signals to provide a second frequency modulated signal, the carrier frequency of said second frequency modulating means being the same as the carrier frequency of said first modulating means; phase shifting means coupled between said source of sine wave signals and one of said modulating means; means for receiving said first frequency modulated signal reflected from a distant object; means for mixing the received signal with said second frequency modulated signal; band pass filter means coupled to the output circuit of said mixing means and tuned to pass one side band component at least as high as the sixth contained in the output from said mixing means; and detector means coupled to said band pass filter means; and means for providing a fixed phase displacement between said transmitted signal and said other frequency modulated signal sufficient to provide in said one side band a first response peak at close range and a second response peak at a substantially greater range with no response peaks therebetween.

United States Patent 1 Erst [ 51 Jan. 29, 1974 F M-CW RADAR RANGE SYSTEMStephen J. Erst. New Haven, Ind.

{73] Assignee: International Telephone and Telegraph Corporation,Nutley, NJ,

[22] Filed: Mar. 12, 1959 [2H Appl. No.: 799,048

[75] Inventor:

Primary ExaminerMalcolm F. l-lubler Attorney, Agent, or Firm.lohn T.OHalloran; Menotti Lombardi, Jr.; Alfred C. Hill EXEMPLARY CLAIM 5. Acontinuous wave radar system comprising: a source of sine wave signals;first means for frequency modulating a carrier frequency with said sinewave signals to provide a first frequency modulated signal; means fortransmitting said first frequency modulated signal; second means forfrequency modulating a carrier frequency with said sine wave signals toprovide a second frequency modulated signal, the carrier frequency ofsaid second frequency modulating means being the same as the carrierfrequency of said first modulating means; phase shifting means coupledbetween said source of sine wave signals and one of said modulatingmeans; means for receiving said first fre' quency modulated signalreflected from a distant object; means for mixing the received signalwith said second frequency modulated signal; band pass filter meanscoupled to the output circuit of said mixing means and tuned to pass oneside band component at least as high as the sixth contained in theoutput from said mixing means; and detector means coupled to said bandpass filter means; and means for providing a fixed phase displacementbetween said transmitted signal and said other frequency modulatedsignal suffcient to provide in said one side band a first response peakat close range and a second response peak at a substantially greaterrange with no response peaks therebetween.

12 Claims, 6 Drawing Figures FHA 35 M00 UL 4 7'0)? SH/FIIP SIN u/ C 106/04 FREQUENCY 001/14 fffl 050/4 l 4 7% 2 1 DITICTflI I 6 FM-CVV RADARRANGE SYSTEM This invention relates generally to radar systems andmethods, and more particularly to radar systems of the frequencymodulated continuous wave type, and to methods utilizing such systems toprovide range discrimination.

Conventional radar systems provide range information, i.e., the locationof a distant object, throughout substantially the entire range of theapparatus. In such conventional radar systems, a transmitted signal isreflected from a distant object and received by a local receiver, thetransmitted signal generally initiating a sweep on a cathode ray tubewith the received signal being displayed on the tube, thus providingcontinuous indication of the range ofthe distant object. Suchconventional radar systems have employed pulse techniques, i.e., thetransmission of short high-frequency pulses, and thus, in order toprovide range information at close distances, extremely short pulsewidths are required. In addition, as indicated, such conventional radarapparatus provides a signal return from all targets within the range ofthe transmitted signal. Furthermore, such conventional pulsed radarapparatus has required complex, heavy and expensive components.

There are applications for radar systems in which it is desired toprovide an output signal when the distant object is at a given rangefrom the transmitting equipment, and at no other time. and it mayfurther be desired that such an output signal be provided when theobject is at an extremely close range from the transmitting apparatus;such functioning is conventionally re ferred to as rangediscrimination". Another type of radar system, which has been referredto as an FM-CW system (frequency modulated-continuous wave) utilizes atransmitted signal which is continuously frequency modulated by a singlesinusoidal signal with a portion of the transmitted signal being mixedwith the received signal. Analysis of the spectral content of theresulting signal output of the mixer reveals that the individual sidebands have an amplitude which is range dependent, Thus, it is known tofilter the output of the mixer to obtain the harmonic or side band ofthe mixed signal which provides the desired range responsecharacteristic. Each harmonic or side band of the signal resulting frommixing the received signal and the transmitted signal has a first majoramplitude peak at a range dependent upon the side band number andfurther has a succession of subsequent amplitude peaks diminishing inmagnitude with increasing range. Thus, in prior FM-CW radar systemsknown to the present applicant, the first detected signal peak of theselected side band is utilized as an output signal, the particular sideband selected thus providing the requisite range discrimination.However, it has been found that in some instances, false or prematuresignals may be pro vided by the second or subsequent amplitude peaks ofthe selected side band; targets. other than the intended target, havingunusually high signal reflectivity (such as large bodies of water) andat a distance from the equipment farther than the range ofthe firstsignal amplitude peak may provide a second or even higher numbered peakof sufficient amplitude to energize the circuitry connected to theoutput of the detector. It is therefore desirable to provide acontinuous wave frequency modulated radar system and method in whichrange discrimination is provided with the unwanted response peak orpeaks which provide erroneous responses in present systems of this typebeing eliminated.

Mathematical analysis, by such means as Bessel functions, of theamplitude variation of the side bands contained in the output of themixer in a CW-FM radar system reveals (and laboratory experimentationdemon strates) that the amplitude variation of each side bandredundantly repeats or mirrors at ranges corresponding to successive ofphase displacement between the transmitted and received signals. Suchmathematical analysis further reveals that the cyclic amplitudecharacteristic of each side band can be said to mirror nega tively,i.e., at what might be referred to as a negative range from thetransmitting equipment. Thus, considering the case of a higher orderside band, such as the tenth, which has its first peak at a considerablerange from the transmitting equipment (in contrast with the carriercomponent which has its first peak at zero range, and the first sideband which has its first peak at very close range), it is seen thatthere is, hypothetically at least, a first negative redundant or mirrorpeak at a negative range from the transmitting equipment, equal to therange of the first positive peak of such higher order side band from thetransmitting equipment, the total distance between these two peaks beingtwice the range ofthe first positive amplitude peak of such higher orderside band. Such higher order side bands have not ordinarily beenemployed in CW-FM radar systems since the targets of interest aregenerally at a much closer range and also since the attenuation ofsignals at the range of the first peak of such higher order side bandsis ordinarily so great that an insufficient output signal level isprovided. I have found, however, that by shifting the phase of thesignal mixed with the received signal with respect to the transmittedsignal by a predetermined amount, the first negative redundant or mirrorpeak of a higher order side band can in effect be shifted into thepositive range region to provide a signal response at a very closerange. Stated differently, by introducing a phase displacement or timedelay between the transmitted signal and the signal which is mixed withthe received signal, the range base line can be said to be shiftedoutwardly until the first response peak of a higher order side bandwhich appears is what was formerly the second major redundant peak ofsuch side band. Thus, by the extraction and detection of a high orderside band and the phase shifting of the transmitted signal with respectto the signal mixed with the received signal, the next redundant peakfollowing the first response peak is that of the former first peak ofthe selected higher order side band (which has been displaced outwardlyin range a corresponding amount) however, propagation losses at thisgreater range results in a signal response from what is now the secondpeak which will always be insufficient to provide false operation of theequipment connected to the radar apparatus.

It is accordingly an object of my invention to provide an improved FM-CWradar system.

Another object of my invention is to provide an improved FM-CW radarsystem which provides range discrimination at close ranges superior tothat provided by prior systems of this type.

A further object of my invention is to provide an improved method forobtaining range discrimination in an FM-CW radar system.

The abovementioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will be best understood by reference to the following descriptionof an embodiment of the invention taken in conjunction with theaccompanying drawings. wherein:

FIG. I is a schematic block diagram showing one embodiment of myinvention;

FIG. 2 is a schematic block diagram showing another embodiment of myinvention;

FIG. 3 shows the amplitude of the first side band (1 in an FM-CW radarsystem;

FIG. 4 shows the amplitude of the detected tenth side band (1 in aconventional CW-FM radar system;

FIG. 5 shows the redundancy or mirroring of the de' tected tenth sideband and is useful in explaining the invention; and

FIG. 6 shows the amplitude of the detected tenth side band in theimproved CW-FM radar system of my invention.

Referring now to FIG. I, in accordance with my invention. I provide afrequency modulated transmitter II which is continuously frequencymodulated by a high purity sinusoidal wave provided by modulator 12. Aswill be hereinafter more fully described, the modulation index must bereasonably high in order to insure the generation of side bands ofmoderately high order while the deviation must be kept at a reasonablylow degree in order to prevent the generation of amplitude modulatedsignals. Since the system will generally be operated in the microwaveregion, the frequency modulated transmitter [I may conventionally be akly stron tube with a suitable carrier frequency being 9000 me and asuitable modulation frequency being I50 kc. As will hereinafter be morefully described, a phase shifting network 13 providing a predeterminedphase shift or phase delay .v is inserted between the modulator l2 andthe frequency modulated transmitter 11. The output oftl'ie frequencymodulated transmitter H is fed to transmitting antenna [4 which may beof any conventional type. the signal being then radiated by thetransmitting antenna 14 to the target and being reflected therefrom andpicked up by the receiving antenna 15.

In accordance with my invention, a local frequency modulated oscillator16 is provided which is likewise continuously frequency modulated by thesine wave provided by the modulator I2, frequency modulated oscillator16 having the same carrier frequency as frequency modulated transmitterII. It will now be seen that the frequency modulated signal provided bythe frequency modulated transmitter l I and transmitted by the antenna14 and the frequency modulated signal provided by the local frequencymodulated oscillator l6 are identical. i.e., having the same carrierfrequency and modulating frequency, being merely phase shifted one withrespect to the other by the amount of the predetermined phase delayprovided by the phase shifting network 13. It will hereinafter bereadily apparent that phase shifting network I3 may equallyadvantageously be inserted between the local frequency modulatedoscillator 16 and the modulator 12.

The signal reflected from the distant target and picked up by thereceiving antenna l5, and the frequency. modulated output signal of thelocal frequency modulator oscillator 16 are mixed in mixer I7, which maybe of the type commonly referred to as a magic T" mixer. Mixing ofthereturned signal as picked up by the receiving antenna 15 and the outputof the local frequency modulator oscillator I6 (which. as indicated, isresponsive to the transmitted signal but phase shifted with respectthereto by a predetermined amount 1) provides in the output circuit ofmixer 17 a signal con taining side bands of frequency 0 and harmonics ofthe modulation frequency. Since the mixer 17 functions as a firstdetector in which the carrier is removed. the carrier component in theoutput signal of mixer 17 actually has only a DC component containingthe doppler frequency. The output signal from mixer 17 may be amplifiedby a conventional video amplifier (not shown) and is in turn fed to asuitable band pass filter 18 which passes the desired higher order sideband (1 I consider that my invention has greatest utility with a sideband at least as high as the eighth being passed by the filter l8, and Ido not believe that any side band lower than the sixth will provide thedesired range discrimination. The output of filter I8 is in turndetected by a second detector 19 with its detected output signal in turnbeing available at output terminal 20 to which other circuitry (notshown) which utilizes the output signal of the system is connected.

Referring now briefly to FIG. 3 in which the amplitude of the detectedsignal is plotted against range. the range-response characteristic ofthe first side band (.I,) of prior conventional FM-C W systems is shownby the curve 21 which includes a first major peak 22 and second andthird and fourth peaks 23, 24 and 25; it will be observed that thesecond and fourth peaks 23 and 25 are the detected negative peaks 23aand 25a. It will further be seen that the peaks 22, 23, 24 and 25 arethose of the Bessel function of the first kind. first order and ofargument M. Horizontal line 26 in FIG. 3 represents a predeterminedthreshold signal of the utilization circuitry connected to the outputterminal 20 of detector 19, i.e., the signal level which will initiatethe desired operational sequence. It is seen that in FIG. 3 only thefirst peak 22 of the range response characteristic 2] is higher than thethreshold signal level 26. However, it will be readily comprehended thata target of high signal reflectivity, such as a large body of water, mayvery possibley result in a returned signal in which the second peak 23or even the third peak 24 would have an amplitude higher than thethreshold signal level 26, thus pro' viding a premature range responseoutput signal.

Referring now to FIGS. 4 and 5 in which there is shown the rangeresponse characteristic of the tenth side band (J it will be seen thatthe tenth side band has a range response characteristic 27 with a firstmajor peak 28 appearing outwardly from the transmitting equipment at arange 29 and having minor peaks 30. 3t, et seq. continuing indiminishing cyclic fashion. As indicated herein before and shown in FIG.5, the rangeresponse amplitude characteristic of each of the side bandsmirrors at a given range outwardly from the transmitting apparatus toprovide redundant peaks as shown at 23a, 30a, 31a, 28b and 30b. Thismirroring or redundancy in the range response characteristic is due tothe fact that there can be only a maximum of of phasedisplacementbetween the transmitted and received signals. In FIG. 5,this maximum 180 phase displacement occurs at a range indicated by thedashed line 35. It will thus be seen that the phase displacement betweenthe transmitted and received signals side of the 180 phase displacementrange 35.

and thus. ignoring attenuation that the amplitudes of the range responsecharacteristic at ranges is the same at the range indicated by the line36 as at the range indicated by the line 36a, i.e.. at the same range oneither side of the 180 phase displace ment range 35, and thus, ignoringattenuation, that the amplitudes of the range response characteristic atranges 36 and 36a will be identical. Therefore, the range responsecharacteristic outwardly between the first 180 phase displacement range35 to the next 180 phase displacement range 35a is the obverse or mirrorof that between the base line 34 and the first 180 phase displacementrange 35 as shown at 31a, 30a and 28a. Outwardly from the second 280phase displacement range, the response characteristic is duplicative ofthat from the base line 34 to the first 180 phase displacement range 35,as shown at 28b, 30b and 31b. Further, as indicated hereinbefore,mathematical analysis reveals hypothetically a negative mirror of eachharmonic at a negative range from the transmitting apparatus and thus,as shown in FlG. 4, the tenth side band may be said to have a negativemirrored or redundant range response characteristic as shown in dashedlines at 32; the negative range response amplitude characteristic of thel0th side band thus has a first major peak 33 at a negative range fromthe transmitting apparatus equal to the positive range 29 of the firstmajor peak 28 and thus, the range between the first major peak 28 andits negative mirror peak 33 is twice the range 29. Referring nowadditionally to FIG. 6, it will be seen that if the l0th side bandrange-response characteristic could be shifted to the right, i.e.,outwardly in range, to bring the first mirror peak 33 of the mirroredrange-response characteristic 32 into the positive region, therange-response peak 28 would likewise be shifted outwardly in range, itbeing observed that there are no other range response characteristicpeaks in the l0th harmonic between the peaks 28 and 33. As another wayof viewing the matter and referring to FIG. 5, if the base line 34 couldbe shifted outwardly to the position shown in the dashed line 34a inFIG. 5, it will be seen that the first response peak now to appear is28a, which formerly was the second major response peak, and that thesecond major response peak is now 28b, which formerly was the thirdmajor response peak. Whichever approach is used. i.e.. shifting thetenth side band range response characteristic, or shifting the rangebase line of the tenth side band, it will be seen that the mirrored peak33 or the second major peak 280 of the l0th side band would provide therequisite close range discrimination with the next peak 28 or 28b beingsufficiently far out in range that attenuation would reduce any signalresponsive thereto to a sufficiently low level as to be of noconsequence.

Turning now to mathematical analysis of the foregoing. the frequencymodulated signa'l transmitted by the transmitting antenna I4 isrepresented by the expression E .4 cos (w! ,8 cos [11+8) where:

A is a constant representing magnitude W =2 rrfl. where j}. is thetransmitter frequency B modulation index (deviation/modulationfrequency) pt 21rf,, where f,, is the modulation frequency I is time 6 6is a phase angle reflected by a target and returned to receiving antenna15 delayed by a time T corresponding to range by the relation T 20 Cwhere D is one way range and C is the velocity of propagation. V

It will be readily understood that the transmitted wave is reflected bya target and returned to receiving antenna 15 delayed by a time Tcorresponding to range by the relation T=2D/C where D is one way rangeand C is the velocity of propagation.

Thus, the received signal is represented by the expression e 0 cos (u-(t T) +,B cos a (r T) 5) where a is dependent upon the signal amplitudeand system gain and represents the summation ofthe losses and gains in asystem such as attenuation, target reflectivity, propagation, losses,etc.

Now, letting the output signal of the local frequency modulatedoscillator 16 be represented by the expression e' b cos (w' (r x) +3 cos,u.(! +5) where w the local oscillator frequency multiplied by 217 andrepresents the phase shift or phase delay inserted by the phase shiftingnetwork 13.

The received signal e and the output e of the local frequency modulatedoscillator 16 are compared in mixer 17 and the resulting voltage isexpressed as t d sin A where A represents the phase difference betweenthe amplitude of the received signal and the am plitude of the output ofthe local frequency modulated oscillator 16, i.e., e and e respectively.Thus:

As indicated above, the carrier frequency of the local frequencymodulator oscillator 16 and that of the frequency modulator transmitter11 are identical, and thus in the above expressions, w equals w, andtherefore:

Let M=2 a sin (C ILX+2 pD/ZC) t dsin w (2D+ Ct/C) M Sin 4! [C 1.1

2 I 0) l which can be further expanded as the sine of the difference oftwo angles.

v=d '1 sin [M sin (at cos own

This can be represented in Bessel series as:

Cox-2 Choosing a component which is the 14" side band, we have v=d2J..(M) [sin a n [cos w C which after detection becomes 1) [1,,(M) lCs-w The above expression is the amplitude of the detected output signalof detector 19, it being recognized that the term cos (-w CxHCl) isdoppler modulation. Furthermore, the term 1,.(M) or, more specifically,1,. [2 [3 sin (C p. .r 2 p. D/2C] will be recognized as having anamplitude dependent upon range Dand the value of the inserted phasedelay time x. It will be seen that the aforementioned mirroring orredundancy is inherent in the foregoing development.

Taking a specific example with the modulating and carrier frequenciesindicated hereinabove, i.e., 9,000 me and 150 kc, it will be assumedthat the range of interest is 200 feet beginning at range 0, and thatall other range-response peaks must be displaced in range to such adegree as to be sufficiently attenuated to be unrecognizable. In orderto insure the generation of side bands of a sufficiently high order,i.e., the 10th side band (1 with .1 being equal to zero at 400 feet when.r 0), B must, as previously indicated, be reasonably high, e.g., 14.5.As further previously indicated, deviation must be kept to a low degreein order to prevent the generation of amplitude modulated signals. Thevalue of X in this case must be (800 feet/C) since the desired functionshould start from zero, and thus, the necessary phase shift or phasedelay is 44, which, it has been found, yields a resultant range-responsecharacteristic as shown in FIG. 5. Here, the unwanted range responsepeak 28 (or 28b) is at a range of 720 feet. and thus will sufferpropagation losses of from (1/0) to l/D4), depending upon the targetreflectivity. Thus, assuming a large target, such as the ocean, therelative amplitude of the 720 foot peak 28 (28h) would be (1/720 80) orH640) of the first or mirror peak 33 (280), or 56.6 db. It will,ofcourse, be readily understood that the above specific example isintended merely to illustrate the functioning ofmy improved system andmethod and does not necessarily represent the best obtainable responsecharacteristic.

It will be readily understood that in accordance with my improved methodof obtaining close-in range response in a (W-FM radar system withdisplacement of unwanted responses outwardly in range so that the resultant attenuation renders them insignificant, it is merely necessaryto provide a phase shift or phase delay between the transmittedfrequency modulated signal and the frequency modulated signal which ismixed with the returned signal. Thus, in the embodiment of FIG. 1 itwill be seen that the phase shifter 13 could be inserted betweenmodulator l2 and frequency modulated transmitter 11, or alternativelybetween modulator l2 and the local frequency modulated oscillator 16.

Referring now to FIG. 2 in which like elements are indicated by likereference numerals, it will be observed that this phase delay may beequally advantageously provided by means of a suitable delay line 34connected between the output of frequency modulated transmitter I1 andmixer 17. Thus, a portion of the transmitted signal from frequencymodulated transmitter ll is fed to mixer l7 to be mixed with thereturned signal received by the receiving antenna l5, this portion beingdelayed in phase by the predetermined amount .r by means of the delaylihe 34.

It will now be seen that my improved FM-CW radar system and the methodof obtaining range discrimination therewith provides, for the firsttime, to the best of the present applicant's knowledge, a closein rangeresponse peak with no other range response peaks appearing within arange to provide unwanted or spurious responses.

While 1 have described above the principles of my invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention.

What is claimed is:

l. A continuous wave radar system comprising: means for providing afrequency modulated signal; means for transmitting said signal; meansfor receiving the transmitted signal reflected from a distant object;means for mixing the received signal with another frequency modulatedsignal identical in frequency and waveform to said transmitted signal;detector means coupled to the output circuit of said mixing means byband pass filter means, said band pass filter means being tuned to passone side band component at least as high as the sixth contained in theoutput signal from said mixing means; and means for providing a fixedphase displacement between said transmitted signal and said otherfrequency modulated signal sufficient to provide in said one side band afirst response peak at close range and a second response peak at asubstantially greater range with no response peaks therebetween.

2. A continuous wave radar system comprising: a source ofsine wavesignals; means for frequency modu lating a carrier frequency with saidsine wave signals; means for transmitting the frequency modulatedsignal; means for receiving the transmitted signal reflected from adistant object; means for mixing the received signal with anotherfrequency modulated signal identical in frequency and waveform to saidtransmitted signal; detector means coupled to the output circuit of saidmixing means by band pass filter means. said band pass filter meansbeing tuned to pass one side band component at least as high as thesixth contained in the output signal from said mixing means; and meansfor providing a fixed phase displacement between said transmitted signaland said other frequency modulated signal sufficient to provide in saidone side band a first response peak at close range and a second responsepeak at a substantially greater range with no response peakstherebetweeni 3. A continuous wave radar system comprising: a sourceofsine wave signals; means for frequency modulating a carrier frequencywith said sine wave signals, said modulating means having a relativelyhigh modulation index and relatively mow deviation thereby generatinghigh order side bands and preventing the genera' tion of amplitudemodulation signals; means for transmitting the frequency modulatedsignal; means for receiving the transmitted signal reflected from adistant object; means for mixing the received signal with anotherfrequency modulated signal identical in frequency and waveform to saidtransmitted signal; band pass filter means coupled to the output circuitof said mixing means and tuned to pass one side band component at leastas high as the sixth contained in the output signal from said mixingmeans; detector means coupled to said band pass filter means; and meansfor providing a fixed phase displacement between said transmitted signaland said other frequency modulated signal sufficient to provide in saidone side band a first responsepeak at close range and a second responsepeak at a substantially greater range with no response peakstherebetween.

4. A continuous wave radar system comprising: a source of sine wavesignal; first means for frequency modulating a carrier frequency withsaid sine wave sig nals to provide a first frequency modulated signal;means for transmitting said first frequency modulated signal; secondmeans for frequency modulating acarrier frequency with said sine wavesignals to provide a second frequency modulated signal, the carrierfrequency of said second modulating means being the same as the carrierfrequency of said first modulating means; means for shifting the phaseof one of said frequency modulated signals with respect to the other;means for receiving said first frequency modulated signal reflected froma distant object; means for mixing the received signal with said secondfrequency modulated signal; band pass filter means coupled to the outputcircuit of said mixing means and tuned to pass one side band componentat least as high as the sixth contained in the output from said mixingmeans; and detector means coupled to said band pass filter means; saidphase shifting means shifting the phase of said one frequency modulatedsignal by a fixed amount sufficient to provide in said one side band afirst response peak at close range and a second response peak at asubstantially greater range with no response p'eaks therebetween.

5. A continuous wave radar system comprisingza source of sine wavesignals; first means for frequency modulating a carrier frequency withsaid sine wave sig nals to provide a first frequency modulated signal;means for transmitting said first frequency modulated signal; secondmeans for frequency modulating a carrier frequency with said sine wavesignals to provide a second frequency modulated signal. the carrierfrequency of said second frequency modulating means being the same asthe carrier frequency of said first modulating means; phase shiftingmeans coupled between said source of sine wave signals and one of saidmodulating means; means for receiving said first frequency modulatedsignal reflected from a distant ogject; means for mixing the receivedsignal with said second frequency modulated signal; band pass filtermeans coupled to the output circuit of said mixing means and tuned topass one side band component at least as high as the sixth contained inthe output from said mixing means; and detector means coupled to saidband pass filter means; and means for providing a fixed phasedisplacement between said transmitted signal and said other frequencymodulated signal sufficient to provide in said one side band a firstresponse peak at close range and a second response peak at asubstantially greater range with no response peaks therebetween;

6. A continuous wave radar system comprising: a source ofsine wavesignals; means for frequency modulating a carrier frequency with saidsine wave signals; means for transmitting the frequency modulatedsignal; means for receiving said signal reflected from a distant object;means for mixing the received signal and a portion of the transmittedsignal; means for shifting the phase of said portion of said transmittedsignal with respect to said transmitted signal; band pass filter meanscoupled to the output circuit of said mixing means and tuned to pass oneside band component at least as high as the sixth contained in theoutput from said mixing means; and detector means coupled to said bandpass filter means; said phase shifting means shifting the phase of saidportion of said transmitted signal by a fixed amount sufficient toprovide in said one side band a first response peak at close range and asecond response peak at a substantially greater range with no responsepeaks therebetween.

7. A continuous wave radar system comprising: a source of sine wavesignals; means for frequency modulating a carrier frequency with saidsine wave signals; means for transmitting the frequency modulatedsignal; means for receiving said signal reflected from a distant object;mixing means having one input circuit coupled to said receiving meansand its other input circuit coupled to said modulating means by timedelay means for mixing the received signal with a portion of thetransmitted signal phase shifted with respect to the transmitted signal;band pass filter means coupled to the output circuit ofsaid mixing meansand tuned to pass one side band component at least as high as the sixthcontained -in the output from said mixing means; and detector meanscoupled to said band pass filter means; said time delay means shiftingthe phase of said portion of said transmitted signal by a fixed amountsufficient to provide in said one side band a first response peak atclose range and a second response peak at a substantially greater rangewith no response peaks therebetween.

8. The method of obtaining radar range discrimination in a continuouswave radar system comprising the steps of: generating a first frequencymodulated signal; transmitting said first frequency modulated signal;receiving said signal reflected from a distant object; mixing thereceived signal with a second frequency modulated signal identical infrequency and waveform to said first frequency modulated signal;filtering the signal resulting from said mixing to pass one side band;at least as high as the sixth; and shifting the phase ofone of saidfirst and second frequency modulated signals with respect to the otherby a fixed amount sufficient to provide in said one side band a firstresponse peak at close range and a second response peak at asubstantially greater range with no response peaks therebetween.

9. The method of claim 8 comprising the additional step ofdetecting thesignal resulting from said filtering l0. The method of obtaining radarrange discrimination in a continuous wave radar system comprising thesteps of: generating sine wave signals; frequency modulating a carrierfrequency with said sine wave signals with a relatively high modulationindex and relatively low deviation to provide a first frequencymodulated signal having high order side bands without amplitudemodulation signals; receiving the transmitted signal reflected from adistnant object; providing a second frequency modulated signal identicalto said first frequency modulated signal; mixing the received signalwith said second frequency modulated signal; filtering the signalresulting from said mixing to pass one side band at least as high as thesixth; detecting the signal resulting from said filtering; and shiftingthe phase from one of said frequency modulated signals with respect tothe other by a fixed amount sufficient to provide in said one side banda first response peak at close range and a second response peak at asubstaitially greater range with no response peaks therebetween.

11. The method of obtaining radar range discrimination in a continuouswave radar system comprising the steps of: generating sine wave signals;frequency modulating a carrier frequency with said sine wave signals toprovide a first frequency modulated signal; transmitting said firstfrequency modulated signal; frequency modulating said carrier frequencywith said sine wave signals to provide a second frequency modulatedsignal; receiving the transmitted signalreflected from a distant object;mixing the received signal with said second fre quency modulated signal;filtering the signal resulting from said mixing to pass one side band atleast as high as the sixth; detecting the signal resulting from saidfiltering; and shifting the phase from one of said frequency modulatedsignals with respect to the other by a fixed amount sufficient toprovide in said one side band a first response peak at close range and asecond response peak at a substantially greater range with no responsepeaks therebetween.

12. The method of obtaining radar range discrimination in a continuouswave radar system comprising the steps of: generating sine wave signals;frequency modulating a carrier frequency with said sine wave signals toprovide a first frequency modulated signal; transmitting said firstfrequency modulated signal; receiving the transmitted signal; mixing thereceived signal and a portion of the transmitted signal; filtering thesignal resulting from said mixing to pass one side band at least as highas the sixth; detecting the signal resulting from said filtering;andphase shifting said transmitted signal portion with respect to thetransmitted signal by a fixed amount sufficient to provide in said oneside band a first response peak at close range and a second responsepeak at a substantially greater range with no response peakstherebetween.

1. A continuous wave radar system comprising: means for providing afrequency modulated signal; means for transmitting said signal; meansfor receiving the transmitted signal reflected from a distant object;means for mixing the received signal with another frequency modulatedsignal identical in frequency and waveform to said transmitted signal;detector means coupled to the output circuit of said mixing means byband pass filter means, said band pass filter means being tuned to passone side band component at least as high as the sixth contained in theoutput signal from said mixing means; and means for providing a fixedphase displacement between said transmitted signal and said otherfrequency modulated signal sufficient to provide in said one side band afirst response peak at close range and a second response peak at asubstantially greater range with no response peaks therebetween.
 1. Acontinuous wave radar system comprising: means for providing a frequencymodulated signal; means for transmitting said signal; means forreceiving the transmitted signal reflected from a distant object; meansfor mixing the received signal with another frequency modulated signalidentical in frequency and waveform to said transmitted signal; detectormeans coupled to the output circuit of said mixing means by band passfilter means, said band pass filter means being tuned to pass one sideband component at least as high as the sixth contained in the outputsignal from said mixing means; and means for providing a fixed phasedisplacement between said transmitted signal and said other frequencymodulated signal sufficient to provide in said one side band a firstresponse peak at close range and a second response peak at asubstantially greater range with no response peaks therebetween.
 2. Acontinuous wave radar system comprising: a source of sine wave signals;means for frequency modulating a carrier frequency with said sine wavesignals; means for transmiTting the frequency modulated signal; meansfor receiving the transmitted signal reflected from a distant object;means for mixing the received signal with another frequency modulatedsignal identical in frequency and waveform to said transmitted signal;detector means coupled to the output circuit of said mixing means byband pass filter means, said band pass filter means being tuned to passone side band component at least as high as the sixth contained in theoutput signal from said mixing means; and means for providing a fixedphase displacement between said transmitted signal and said otherfrequency modulated signal sufficient to provide in said one side band afirst response peak at close range and a second response peak at asubstantially greater range with no response peaks therebetween.
 3. Acontinuous wave radar system comprising: a source of sine wave signals;means for frequency modulating a carrier frequency with said sine wavesignals, said modulating means having a relatively high modulation indexand relatively mow deviation thereby generating high order side bandsand preventing the generation of amplitude modulation signals; means fortransmitting the frequency modulated signal; means for receiving thetransmitted signal reflected from a distant object; means for mixing thereceived signal with another frequency modulated signal identical infrequency and waveform to said transmitted signal; band pass filtermeans coupled to the output circuit of said mixing means and tuned topass one side band component at least as high as the sixth contained inthe output signal from said mixing means; detector means coupled to saidband pass filter means; and means for providing a fixed phasedisplacement between said transmitted signal and said other frequencymodulated signal sufficient to provide in said one side band a firstresponse peak at close range and a second response peak at asubstantially greater range with no response peaks therebetween.
 4. Acontinuous wave radar system comprising: a source of sine wave signal;first means for frequency modulating a carrier frequency with said sinewave signals to provide a first frequency modulated signal; means fortransmitting said first frequency modulated signal; second means forfrequency modulating a carrier frequency with said sine wave signals toprovide a second frequency modulated signal, the carrier frequency ofsaid second modulating means being the same as the carrier frequency ofsaid first modulating means; means for shifting the phase of one of saidfrequency modulated signals with respect to the other; means forreceiving said first frequency modulated signal reflected from a distantobject; means for mixing the received signal with said second frequencymodulated signal; band pass filter means coupled to the output circuitof said mixing means and tuned to pass one side band component at leastas high as the sixth contained in the output from said mixing means; anddetector means coupled to said band pass filter means; said phaseshifting means shifting the phase of said one frequency modulated signalby a fixed amount sufficient to provide in said one side band a firstresponse peak at close range and a second response peak at asubstantially greater range with no response peaks therebetween.
 6. Acontinuous wave radar system comprising: a source of sine wave signals;means for frequency modulating a carrier frequency with said sine wavesignals; means for transmitting the frequency modulated signal; meansfor receiving said signal reflected from a distant object; means formixing the received signal and a portion of the transmitted signal;means for shifting the phase of said portion of said transmitted signalwith respect to said transmitted signal; band pass filter means coupledto the output circuit of said mixing means and tuned to pass one sideband component at least as high as the sixth contained in the outputfrom said mixing means; and detector means coupled to said band passfilter means; said phase shifting means shifting the phase of saidportion of said transmitted signal by a fixed amount sufficient toprovide in said one side band a first response peak at close range and asecond response peak at a substantially greater range with no responsepeaks therebetween.
 7. A continuous wave radar system comprising: asource of sine wave signals; means for frequency modulating a carrierfrequency with said sine wave signals; means for transmitting thefrequency modulated signal; means for receiving said signal reflectedfrom a distant object; mixing means having one input circuit coupled tosaid receiving means and its other input circuit coupled to saidmodulating means by time delay means for mixing the received signal witha portion of the transmitted signal phase shifted with respect to thetransmitted signal; band pass filter means coupled to the output circuitof said mixing means and tuned to pass one side band component at leastas high as the sixth contained in the output from said mixing means; anddetector means coupled to said band pass filter means; said time delaymeans shifting the phase of said portion of said transmitted signal by afixed amount sufficient to provide in said one side band a firstresponse peak at close range and a second response peak at asubstantially greater range with no response peaks therebetween.
 8. Themethod of obtaining radar range discrimination in a continuous waveradar system comprising the steps of: generating a first frequencymodulated signal; transmitting said first frequency modulated signal;receiving said signal reflected from a distant object; mixing thereceived signal with a second frequency modulated signal identical infrequency and waveform to said first frequency modulated signal;filtering the signal resulting from said mixing to pass one side band;at least as high as the sixth; and shifting the phase of one of saidfirst and second frequency modulated signals with respect to the otherby a fixed amount sufficient to provide in said one side band a firstresponse peak at close range and a second response peak at asubstantially greater range with no response peaks therebetween.
 9. Themethod of claim 8 comprising the additional step of detecting the signalresulting from said filtering.
 10. The method of obtaining radar rangediscrimination in a continuous wave radar system comprising the stepsof: generating sine wave signals; frequency modulating a carrierfrequency with said sine wave signals with a relatively high modulationindex and relatively low deviation to provide a first frequencymodulated signal having high order side bands without amplitUdemodulation signals; receiving the transmitted signal reflected from adistnant object; providing a second frequency modulated signal identicalto said first frequency modulated signal; mixing the received signalwith said second frequency modulated signal; filtering the signalresulting from said mixing to pass one side band at least as high as thesixth; detecting the signal resulting from said filtering; and shiftingthe phase from one of said frequency modulated signals with respect tothe other by a fixed amount sufficient to provide in said one side banda first response peak at close range and a second response peak at asubstaitially greater range with no response peaks therebetween.
 11. Themethod of obtaining radar range discrimination in a continuous waveradar system comprising the steps of: generating sine wave signals;frequency modulating a carrier frequency with said sine wave signals toprovide a first frequency modulated signal; transmitting said firstfrequency modulated signal; frequency modulating said carrier frequencywith said sine wave signals to provide a second frequency modulatedsignal; receiving the transmitted signalreflected from a distant object;mixing the received signal with said second frequency modulated signal;filtering the signal resulting from said mixing to pass one side band atleast as high as the sixth; detecting the signal resulting from saidfiltering; and shifting the phase from one of said frequency modulatedsignals with respect to the other by a fixed amount sufficient toprovide in said one side band a first response peak at close range and asecond response peak at a substantially greater range with no responsepeaks therebetween.
 12. The method of obtaining radar rangediscrimination in a continuous wave radar system comprising the stepsof: generating sine wave signals; frequency modulating a carrierfrequency with said sine wave signals to provide a first frequencymodulated signal; transmitting said first frequency modulated signal;receiving the transmitted signal; mixing the received signal and aportion of the transmitted signal; filtering the signal resulting fromsaid mixing to pass one side band at least as high as the sixth;detecting the signal resulting from said filtering; and phase shiftingsaid transmitted signal portion with respect to the transmitted signalby a fixed amount sufficient to provide in said one side band a firstresponse peak at close range and a second response peak at asubstantially greater range with no response peaks therebetween.